The present invention relates to a reflective optical encoder device for optically detecting an angular or linear speed or position of a movable member by applying a laser beam to a series of pits or depressions burned in an encoder disc that is coupled to the movable member and then reading a laser beam modified and reflected by the series of pits.
Optical encoder devices such as rotary optical encoder devices are widely used in the art. A rotary optical encoder device has an input shaft directly coupled to a rotatable drive shaft of a movable member whose angular speed or position is to be measured. The angular speed or position of the drive shaft is optically detected by a light beam, such as a laser beam, applied to and modified somehow by an encoder disc that is coupled to the input shaft.
Rotary optical encoder devices are roughly grouped into two classes. In one class, a rotary encoder disc has a multiplicity of slits or pits which are detected by a light beam passing through the slits or pits and modified thereby during rotation of the rotary encoder disc. According to the other group, slits or pits defined in a rotary encoder disc are detected by a light beam reflected and modified by the slits or pits while the rotary encoder disc is rotating.
More specifically, a reflective rotary optical encoder comprises a rotary encoder disc having ring-shaped encoder tracks in the form of a series of slits or pits defined in a certain circumferential pattern by the process of manufacturing optical discs such as video discs and compact discs. A laser beam which is generated typically by a semiconductor laser is applied to the encoder tracks of the rotary encoder disc. The encoder tracks are thus read by detecting a laser beam reflected from the encoder disc and modulated in intensity by the encoder tracks, and converting the detected laser beam to an electric signal.
According to one conventional reflective rotary optical encoder device, such an electric signal has been generated by a laser beam pickup which adds all output signals from a four-sector photodiode that is also used to produce a focus servo signal. The laser beam pickup of the conventional reflective rotary optical encoder device cannot however generate two signals which are out of phase with each other by a phase angle of 90.degree. for detecting the direction in which the the rotary encoder disc rotates.